1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods of and apparatus for packaging an integrated circuit.
2. Description of the Related Art
Many current integrated circuits are formed as multiple die on a common wafer. After the basic process steps to form the circuits on the die are complete, the individual die are cut from the wafer. The cut die are then usually mounted to structures, such as circuit boards, or packaged in some form of enclosure.
One frequently-used package consists of a substrate upon which a die is mounted. The upper surface of the substrate includes electrical interconnects. The die is manufactured with a plurality of bond pads. A collection of solder bumps are provided between the bond pads of the die and substrate interconnects to establish ohmic contact. After the die is mounted to the substrate, a lid is attached to the substrate to cover the die. Some conventional integrated circuits, such as microprocessors, generate sizeable quantities of heat that must be ferried away to avoid device shutdown or damage. The lid serves as both a protective cover and a heat transfer pathway.
To provide a heat transfer pathway from the integrated circuit to the lid, a thermal interface material is placed on the upper surface of the integrated circuit. In an ideal situation, the thermal interface material ideally fully contacts both the upper surface of the integrated circuit and the portion of the lower surface of the lid that overlies the integrated circuit. Conventional thermal interface materials include various types of pastes, and in some cases, a metal. More recently, designers have begun to turn to indium as a thermal interface material.
The attachment of a lid to a die substrate involves a complex choreography of steps. The thermal interface material must be applied to the die. An adhesive must be applied to the substrate and cured in such a way that does not produce unwanted irregularities in the thickness or wetting of the thermal interface material. The lid must be attached to the substrate so that the tilt of the lid relative to the die is within acceptable tolerances. High tilt can lead to nonuniformities in thermal interface material thickness, which can produce poor heat transfer characteristics.
Indium as a thermal interface material presents certain challenges. A consistent metallurgical bond between the integrated circuit and the indium, and in turn, between the indium and the package lid is desirable in order to provide a uniform thermal resistance of heat transfer pathway away from the integrated circuit and into the lid. Achieving the necessary wetting of indium is not a trivial matter. Furthermore, the aforementioned tilt of the lid may be impacted by thermally-induced movement of the lid adhesive during steps to bond the indium.
Current techniques for establishing metallurgical bonding between a lid, an integrated circuit and the indium thermal interface material sandwiched therebetween involves the use of a flux film applied to both the upper surface of the integrated circuit and the upper surface of the indium thermal interface material. A subsequent reflow process produces a melting followed by a solidification of the indium material which produces the metallurgical bonding. In an ideal process, the flux would be completely displaced during the reflow such that a relatively homogeneous layer of indium remains after reflow. However, conventional indium thermal interface material is applied as a solid sheet or preform. During reflow, the edges of the indium preform can solidify and create a physical barrier that blocks the escape routes for flux remnants. Trapped flux remnants can result in the formation of voids in the indium. Voids in the indium represent areas of higher thermal resistance. Depending on the location of these “hot spots,” device performance can be adversely impacted.
One conventional type of package lid has a concave underside that faces toward an indium thermal interface material layer. During reflow, the concavity can produce the aforementioned peripheral solidification and resulting entrapment of flux remnants. Conventional concave lids, as well as planar lids, utilize a gold film to facilitate wetting of indium during reflow. Conventional gold films are typically square and have footprints that generally track the footprints of the underlying die. An issue associated with conventional gold films is the possibility that there will insufficient indium near the corners of the die after reflow. With insufficient indium at the corners, stresses applied to the indium can cause significant cracking of the indium preform after the repeated thermal cycling that occurs during normal device operation. The cracks create areas of heightened thermal resistance and thus hot spots.
Another anomaly sometimes observed in indium thermal interface materials is the appearance of a glob of indium that protrudes laterally from the bulk preform after reflow. Such globs tend to appear randomly. A glob, if large enough and in the right spot, may touch and damage or short external devices, such as capacitors.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.